The modes of action of pesticidal chemicals and their metabolism in vertebrate cell cultures are in most cases completely unknown. Several toxicity screens have been performed in the past, but the usefulness of these studies is severely limited by the restriction of attention to cell growth parameters and by the lack of knowledge or control over the actual chemical changes in the cell environment. In order to provide a basis for the future design of cell toxicity screens, we have emphasized a shift in attention to the toxicities of chemicals to selected cellular activities. A simple screen based on inhibition of amino acid and uridine incorporation into protein and RNA in HeLa cells has been applied to 30 representative pesticides, and it is proposed to determine the general inhibitory mechanisms behind the observed discrimination. Thus, pesticide interference with nutrient transport, cellular ATP levels, and macromolecular synthesis will be studied and evaluated. Since the observed effects of added chemicals on any cellular activity will depend on the amount taken up by the cell and chemical changes in the growth medium, these parameters must be known to determine the relative tocicities of the chemicals. We have proposed to measure the cellular uptake of several representative pesticides, to investigate the inactivation of growth medium components, and to determine the chemical and physical stabilities of pesticide/media over 24-hour periods. This information will be used to assess the relative toxicities of selected pesticides to the biochemical activities of mammalian cell cultures. o